A conventional folding shaft of an umbrella generally includes three or four sections which are retractably connected to each other in a sequence such that when the umbrella is not in use, the shaft can be folded to be a short shaft. The foldable shaft is cooperated with an opening device which includes a spring arranged to be extended through the sections of the shaft. In order to smoothly extend or fold the shaft, the inner diameter of each section has to be controlled so as to ensure smooth operation while the opening device can be installed therein. However, a gap is defined between the sections and affects the operation of the folding shaft. Conventional folding shaft of umbrellas generally includes sections each have a circular cross section and the friction between the sections is significant. If the gaps between the circular sections are made large to reduce the friction and accommodate the spring of the opening device, the spring might not be installed in stable condition.
The present invention intends to provide a folding shaft for umbrellas and the sections of the folding shaft includes sections with polygonal shape and sections with circular section, the sections with polygonal shape and sections with circular section are retractably connected to each other in an alternative sequence so as to obtain minimum gap between sections.